This invention relates to a pre-treating solution for electroless plating functioning as adhesion accelerating agent, particularly used for producing printed wiring boards.
Printed wiring boards are produced in the order of drilling of holes in a copper-clad laminate or a laminate having no copper thereon, electroless plating, pattern formation, removal of resist, etc.
Pretreatment step for electroless plating comprises conducting degreasing and washing of a substrate after drilling holes, softly etching copper surface on the substrate, washing with dilute sulfuric acid, dipping in a sensitizer containing palladium chloride/stannous chloride, etc., removing metallic impurities such as an .alpha.-stannic acid (H.sub.2 SnO.sub.3) layer attached to a substrate surface and inner walls of holes with an adhesion accelerating agent, and depositing copper using an electroless copper plating solution. As the adhesion accelerating agent, there have been used an aqueous solution containing hydrochloric acid, or an aqueous solution containing hydrochloric acid and oxalic acid (Japanese Patent Examined Publication (JP-B) No. 53-34103), an aqueous solution containing borofluoric acid, and an aqueous solution containing sodium hydroxide and ethylene-diaminetetraacetic acid (EDTA) (Japanese Patent Unexamined Publication (JP-A) No. 51-8127).
But these adhesion accelerating agents have various problems. For example, in the case of using the adhesion accelerating agent containing borofluoric acid as a major component, when a pattern is formed using an alkaline ammonium etchant solution according to a pattern plating method, wedge type etching corrosion takes place at the boundary between a copper foil on the substrate and plated copper, resulting in causing a problem in reliability on electrical properties. Further, there is also a defect in that adhesiveness of plated copper to glass cloth portions is poor. As a result, there is a problem of a high generating rate of blow holes in the solder pick-up test.
In the case of the adhesion accelerating agent of JP-B 53-34103, there are problems in that a usable range of concentration of copper dissolved from a substrate copper is narrow, the life of solution is short, and when standing time in air becomes 15 seconds or longer, rust is easily produced on the substrate copper surface. As a result, surface roughness of plating after electroless plating takes place.
In the case of the adhesion accelerating agent of JP-A 51-8127, there is a problem in that a plated layer is easily peeled off from the substrate copper foil.